how to disable a treenode before it had been checked?

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VB

Hello!I use a treeview with some nodes,I want that some of these nodes shoule be disable like enable = false before they were checked.
I use codes like below,in the treeview Before_Checked events but I found the node still can be click ,could anyone tell me how to change it and let it work?THX a lot!

Private Sub aTree_BeforeCheck(ByVal sender As Object, ByVal e As System.Windows.Forms.TreeViewCancelEventArgs) Handles aTree.BeforeCheck
        Dim a As TreeNode
        a = aTree.Nodes(0).Nodes(1)
        a = e.Node
        If e.Node.ForeColor = Color.Gray Then
            e.Cancel = True
        End If
    End Sub